Up to now, grid services, such as the provision of rotating mass and reactive power, have been managed virtually secondarily by large power plants, with large turbosets, being in operation. As a result of the preferred feed of electric current from photovoltaic power plants and wind power plants, which is pursued in certain countries, these power plants are squeezed out of the market or taken off the grid, at least for a time, and therefore these can no longer provide these grid services. Photovoltaic power plants and wind power plants, on account of their technical features, are able to provide no, or only little, reactive power, short circuit power and rotating mass for ensuring grid stability. This problem will be magnified still more in the future as the percentage of these renewable energies increases further.
Furthermore, owing to the lack of storage possibilities for the renewable electric current, the requirement also exists for a demand for an ensured fossil reserve, capable of quick starting, for when the demand for electric current cannot be covered via the renewable energies. This demand for real power and its quick change can preferably be covered via gas turbine power plants which are relatively favorable and quick to start, which power plants can be operated comparatively economically especially in the case of a low number of utilization hours. Their economic efficiency is further improved if these can provide the required grid services (reactive power and short circuit power and also fixedly coupled rotating masses) without real power having to be provided.
If it is to become possible for the gas turbine power plant to provide reactive power, etc., without real power being delivered to the grid at the same time, the generator which is synchronized with the grid has to be disconnected from the gas turbine via a clutch. In the case of gas turbine power plants which are based on large single-shaft, axial flow gas turbines this leads to the problem that the generator is then not available for a restarting process of the gas turbine (the generator is “converted” for this type of gas turbine with the aid of a starting converter for the starter motor), since, being synchronized with the grid, it is required as a rotating phase changer for grid stabilization while the gas turbine is kept in turning operation only at low rotational speed or, if necessary, is stationary (turning operation: turning of the rotor during the cooling down phase in order to avoid bowing of the hot rotor).
In previous designs, for starting a gas turbine power plant which is provided with a clutch between a large single-shaft, axial flow gas turbine and generator, the generator had to be disconnected from the grid and decelerated to the rotational speed of the gas turbine in order to be able to couple the generator and the gas turbine for the starting process with the result that the grid services had to be interrupted for a certain and possibly critical period.
Up to now, there have been no practical solutions for starting a large single-shaft, axial flow gas turbine which is independent of the generator (or of a separate starter motor connected on the generator side). Existing solutions for small gas turbines, which for example connect a starter motor to a transmission provided for the generator connection, are not provided for these gas turbines. Against the further possibility of greatly enlarging the existing hydraulic turning gear (e.g. on the basis of a Pelton wheel which is driven via lubricating oil provided from the high-pressure lift oil pump) of the large single-shaft, axial flow gas turbine and therefore also utilizing such for starting the gas turbine, is the plant-technical cost and the inefficiency of such a system, which in turn would correspondingly increase the power consumption during startup of the gas turbine and entail further costs (e.g. enlargement of a possibly existing black start diesel).